1. Field of the Invention
This invention relates to calibrating weighing installations.
2. Description of Related Art
Installations such as weigh platforms or vessels (such as silos, tanks and hoppers) which are used in industrial processes for information and process control purposes usually have a working configuration of at least one working load cell, a support for a fixed side of the load cell, and a load mounted on a load-bearing side of the load cell. The load is usually transmitted to the working load cell through a vessel support bracket or other special bearing surface provided on the vessel, adapted to carry its weight and the weight of any contents. Typically, three or four working load cells, with corresponding supports and loads, are used in any one installation, so the aggregate load is the sum of all the separately measured loads. Some platforms and vessels use a mechanical arm system to transfer their weight to a single load cell. The term vessel will be used herein for convenience to include all receptacles, containers, platforms or the like for supporting whatever is to be weighed by a weighing installation.
Such installations need to be calibrated and re-calibrated from time to time, because of drift or creep in the electrical or mechanical properties of the load cells, or in the tare of the weigh platform or vessel. It is not usually practicable or good practice to remove the load cell or cells from the installation for re-calibration, because of the substantial impact this will make on the availability of the installation. In an industrial process, it normally would mean production downtime. Instead, the usual method adopted is to load the installation with a standard weight, which is compared with the total weight indicated by the working load cells. This indicated total weight is conventionally the single integrated or summated output of all load cells combined. Conventional weighing calibration is the calibration of the whole system, including all load cells, sources and measures of applied excitation voltage to the load cells, and meters for interpreting the corresponding load cell output signals. A necessary consequence is that, if any part of this system fails or is adjusted or replaced, the whole system must be recalibrated.
The use of standard weights can be time-consuming, dangerous and inaccurate. Considering that this invention is particularly devised for use with weight vessels typically ranging from 250 kg to over 100,000 kg capacity, the limitations of using dead weights of known value, especially at the higher end of this range, will be clear. In the case of tanks, metered quantities of water can be used, calculating weights from the known volumes and density, but it is known that flow meters are of limited accuracy. Loading a vessel with known weights may not be possible over the full capacity of the vessel, so that calibration of the maximum load is dependent upon extrapolation from lower loads, but the calibration curve may not in fact be linear, uniform or regular.
However, although such methods are conventionally used, they still imply a substantial interruption in the normal operation of the weighing installation, which can be especially costly if it is normally in use in a continuous industrial process.
It is an object of the invention to provide an alternative system and method for calibrating weighing installations of the above kind, which enable calibration to be carried out quickly and accurately.
The present invention concerns aspects of the system for calibrating weighing installations described below. The scope of the invention extends to all novel aspects of the system, including methods of putting it into effect, whether individually or in combination with any of the other features disclosed herein.
More specifically, in one aspect of the invention, a system for calibrating weighing installations of the aforementioned kind comprises portable apparatus movable between installations to be calibrated, and fixed apparatus at each installation to be calibrated, wherein the portable apparatus includes a reference load cell and a fluid ram, the fixed apparatus includes anchorage means fast with the working load cell support, and the reference cell and the ram are removably connectable between the anchorage means and the load-bearing side of the working load cell to apply calibrating loads to the working load cell in its working configuration.
The fixed elements of the calibration system need be no more than the anchorage means only, at each working load cell support. The working load cells, which generally tend to be shear beams of one design or other, normally rest on load plates. The anchorage means can with advantage be incorporated into each load plate. Alternatively, the anchorage means can be provided on the solid base to which the load plates themselves are fastened. The anchorage may be mechanical, or electromagnetic, in which case a ferromagnetic material may be provided as the anchorage means, to be engaged by electromagnets in the portable apparatus.
The anchorage means are desirably close to the working load cells. It is in practice unlikely that the calibrating loads will be applied directly to the working load cell, because its load-bearing side is already carrying the weigh vessel. The calibrating load is accordingly applied to the weigh vessel itself, and it is generally convenient to apply those loads to the parts of the structure that are designed to take and transmit loads, which will normally be adjacent the point at which the weigh vessel is mounted on the working load cell, for example at an existing mounting bracket.
The portable elements in the system include the fluid ram, normally a hydraulic ram, and the reference load cell, and may also include supplementary fixings, for example a cradle to fasten to the anchorage means and hold the ram and reference cell in a proper position to exert calibrating loads on to the working load cell. Preferably, position adjustment means are included in the portable elements, to permit the reference cell to be properly positioned in relation to the working cell in different weighing installations, in which the anchorage means may not be identically positioned in relation to the working cells.
The reference cell should be accurate and may be periodically standardised by secondary referents traceable to an appropriate ultimate standard.
In a method in accordance with the invention, corresponding to the use of the system described above, the reference cells and the ram are connected between the anchorage means and the load-bearing side of the working load cell of each installation in turn, and calibrating loads are applied to each working load cell in its working configuration.
In the common case where a weighing installation has more than one working load cell, the portable apparatus may have as many reference load cells and fluid rams as are necessary to apply calibrating loads to each one of the plurality of working load cells in its working configurationxe2x80x94usually simply one reference cell and one ram for each working load cell. This enables faster calibration. It also enables calibration of each load cell selectively as the sole cell to which a load is applied, or while other loads are applied to other cells, or, in a more natural working condition, while similar loads are applied to the other cells, so that the loads are balanced between the working cells. Accordingly, the portable apparatus may include means for supplying fluid under pressure to the fluid ram associated with each one of the plurality of reference load cells, means for recording each calibrating load applied thereto as measured by the reference load cells, and means for recording the corresponding output of the working load cells to provide a calibration record. The method of the invention may be applied accordingly,
Likewise, the system of the invention may include control means for varying the pressure of the fluid supplied to the rams whereby to control the force exerted by a given ram on the corresponding reference and working load cells in a sequence of calibration steps for each working load cell, and may include switch means for diverting fluid to the ram or rams associated with each of the plurality of reference load cells in turn, and for selecting the outputs of the corresponding load cells for recording, with corresponding applications of the method of the invention.